For several years, scientists have been working to determine why so many male smallmouth bass in the Potomac River basin have immature female egg cells in their testes - a form of intersex. They are closer to finding an answer.

Research by the U.S. Geological Survey (USGS) shows that a high incidence of intersex occurs in the Potomac watershed at sites where farming is most intense and where human population density is highest. The study also shows the greatest prevalence of this form of intersex, known as testicular oocytes (TO), occurs in the spring, just before and during the spawning season.

These results are published in the current edition of the Journal of Aquatic Animal Health.

"We collected smallmouth bass from the Shenandoah, the South Branch of the Potomac, and out of the basin for comparison," said USGS scientist Vicki Blazer, who led the study. "The fish from the sites with the highest human population density and the most farming had the highest incidences of intersex," said Blazer. "On the Shenandoah, rates of intersex were highest, ranging from 80-100 percent intersex."

Out of the Potomac basin, the most densely populated heavily farmed site had bass with a TO rate of 75 percent, where less habited sites had 14-35 percent of male bass with TO. Sites along the South Branch of the Potomac ranged from 47-77 percent; again the higher percents corresponding with increased farming and human population.

Seasonal comparisons are also striking. In the study, the USGS sampled six sites. At every site sampled, the incidence of male bass with TO was significantly higher during the spring pre-spawn to spawning period, ranging from 69-100 percent, compared to the summer post-spawn period, when it ranged from 25-67 percent.

The reproductive anomalies in the Potomac's smallmouth bass population are not readily apparent on gross examination of an affected fish -- they were discovered by accident. In 2003, scientists investigating massive fish kills and widespread lesions found many individuals with TO while looking at tissues from the testes of male fish under the microscope.

A prevalence of intersex is not unique to the Potomac basin, nor is it unique to smallmouth bass. It has been documented in other wild fish populations including spot-tail shiners in the St. Lawrence River, white suckers in Colorado, shovelnose sturgeon in the Mississippi, white perch from the Great Lakes, roach fish in the U.K and Denmark, sharp-tooth catfish in South Africa, three-spine stickleback in Germany, and barbel in Italy. It has also been noted in marine and estuarine fishes in Japan, the UK and the Mediterranean.

At many of these places, it has been associated with known or suspected endocrine disrupting compounds in wastewater effluent, which are not removed during standard sewage treatment, and in runoff from farming operations. These compounds can include estrogen from birth control pills and hormone replacements, pesticides and fertilizers used on crops, and hormones from livestock operations.

Scientists are continuing to assess the extent of TO in bass in the Potomac River system. They are examining samples collected at reference sites within and outside of the drainage basin to determine a background prevalence of TO for both smallmouth and largemouth bass, and to identify potential causes. They are also assessing the reproductive and general health of fish at sites with high and low prevalence of TO, and evaluating land use in risk assessment.

Reporters: The article "Intersex (Testicular Oocytes) in Smallmouth Bass from the Potomac River and Selected Nearby Drainages," is available by email dnoseral@usgs.gov

Studies of fish health are part of the USGS Chesapeake Bay studies, which provide integrated science for improved understanding and management of the Bay ecosystem. More information about USGS Chesapeake Bay studies, and a soon to be released report "USGS Circular 1316, "Synthesis of USGS Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management," can be found at http://chesapeake.usgs.gov/.

USGS provides science for a changing world. For more information, visit www.usgs.gov.